A gas turbine engine contains a compressor in fluid communication with a combustion system that often contains a plurality of combustors. The compressor raises the pressure of the air passing through each stage of the compressor and directs it to the combustors where fuel is injected and mixed with the compressed air. The fuel and air mixture ignites and combusts creating a flow of hot gases that are then directed into the turbine. The hot gases drive the turbine, which in turn drives the compressor, and for electrical generation purposes, can also drive a generator.
Most combustion systems utilize a plurality of fuel injectors for staging, emissions purposes, and flame stability. Fuel injectors for applications such as gas turbine combustion engines direct pressurized fuel from a manifold to the one or more combustion chambers. Fuel injectors also function to prepare the fuel for mixing with air prior to combustion. Each fuel injector typically has an inlet fitting connected either directly or via tubing to the manifold, a tubular extension or stem connected at one end to the fitting, and one or more spray nozzles connected to the other end of the stem for directing the fuel into the combustion chamber. A fuel passage (e.g., a tube or cylindrical passage) extends through the stem to supply the fuel from the inlet fitting to the nozzle. Appropriate valves and/or flow dividers can be provided to direct and control the flow of fuel through the nozzle and/or fuel passage.
The fuel passage, also referred to as fuel supply member, a fuel feed strip or macrolaminate strip, is typically supported at each end thereof in a cavity within the stem. In a typical fuel injector, the stem is exposed to the high temperatures of the combustor and undergoes thermal expansion in response to the higher temperatures. The fuel feed strip, being cooled by the fuel flowing internally thereto, generally undergoes thermal expansion to a lesser degree than the stem. This difference in thermal expansion can result in undesirable stresses being placed on the fuel feed strip and/or stem. Accordingly, fuel feed strips typically have some axial flexibility to mitigate such stresses.
An example of a fuel feed strip supported at each end within a chamber of a stem is disclosed in U.S. Pat. No. 6,711,898 to Laing et al. The single fuel feed strip (fuel passage) contained in the hollow stem of the injector has a convoluted shape that provides some axial flexibility to allow axial expansion and contraction of the fuel feed strip in response to thermal expansion and/or contraction of the stem and/or fuel feed strip itself.
Of particular concern in the design of any component of a gas turbine engine, and in particular the fuel feed strip, is both high and low cycle fatigue. Low cycle fatigue generally occurs due to thermal expansion and contraction of engine components during operation, as just described. High cycle fatigue generally occurs when resonance or vibration modes are excited by driving frequencies inherent in the operation of the engine. For example, shaft rotation imbalance can produce driving frequencies between about 200 to about 300 Hertz (Hz). Driving frequencies due to combustion rumble can be in the range of about 300 Hz to about 800 Hz. Fuel pump pulsations can produce driving frequencies in the range of 1200 Hz. Blade passing frequencies can be upwards of 1200 Hz.
Prior art fuel injectors have incorporated devices and designs, such as that shown in U.S. Pat. No. 6,038,862, to address the issue of high cycle fatigue. Typically, such devices are intended to damp vibration of the parts to avoid resonance. However, such devices can be complex and require additional parts which can resonate themselves. Further, many such devices must be installed prior to assembly of the fuel injector and are not easily serviced. Some designs can restrict movement of the fuel feed strip in response to thermal expansion of the stem and/or strip and thereby induce undesirable stresses in the assembly.
Another approach has been to alter the natural frequency, also referred to herein as resonant frequency, of the parts. In general, reinforcing ribs and/or additional structure is provided to increase the natural frequency of the part above the anticipated driving frequencies of the turbine. While effective in many applications, the additional structure can be bulky and also tends to increase the stiffness of the parts which can be undesirable in applications where flexibility of the part is desired or necessary. Further, in the event a resonant driving frequency occurs, such approach does not provide damping to dissipate energy from the assembly.
Still another approach has been to alter the natural frequency of the part by shaping the part such that its natural frequency is above the maximum driving frequency the part will experience. For example, U.S. Pat. No. 6,098,407 discloses a fuel injector including a fuel supply tube that is coiled into a 360 degree spiral shape. Ideally, the curvature of the tube is such that the tube's natural frequency is well above the maximum vibratory frequency that the tube will experience during engine operation. Again, while effective for many applications, such approach does not provide damping to dissipate energy from the assembly and thus if a resonant driving frequency occurs, the fuel feed strip can be damaged.